add worklog
interpret assignments and function call statements

test case that local environments get popped again
correct scoping

There are exactly two scopes where identifiers are looked up: the
current local and the global scope.  Identifiers are never looked up in
any other scope.
put example programs to separate subfolder
interpret blocks, while loops and if-then-else
add Either to Spl, to model return values
pVarDeclaration: type and identifier must be separated
handle return values
simplify spec, silence compiler warnings